4.17 PPP - Connection Of Computers Via Serial Interface

With OPT_PPP='yes' a computer can be connected via its serial interface. This may be useful i.e. for connecting computers to a network that does not have a network interface card. The computer connected to the serial interface is called client PC below.

PPP_DEV

Specify fli4l's serial interface the client PC is connected to. Possible values are:

'com1' COM1 port (lower cases only!)
'com2' COM2 port (lower cases only!)

PPP_SPEED

Set transfer rate here (bit/sec). 38400 is supported by older interfaces too. Problems may occur when using rates above this.

    Example: PPP_SPEED='38400'

PPP_IPADDR PPP_PEER

PPP_IPADDR holds fli4l's IP address on the COM port, i.e. '192.168.4.1'. In variable PPP_PEER the IP address of the client PC is set, i.e. '192.168.4.2'.

PPP_NETWORK PPP_NETMASK

PPP_NETWORK holds the network used and variable PPP_NETMASK the netmask. These two variables are used by the extra package 'samba_lpd'.


Important: Consider the following:

  1. Those IP addresses may not originate from the network address range of the ethernet LANs. Point-To-Point-Configuration must use a separate network address range!

  2. For the Client PC's connection to the Internet this mini-PPP-network has to be masked in the same way as the LAN. The net has to be entered additionally in MASQ_NETWORK (see next chapter) as well.

  3. The client PC should be added to the host table of the DNS server.

Causes:

If telnet or ftp should be used from the client PC to the fli4l router the daemons concerned on fli4l do a reverse DNS lookup to resolve the client PC. If it is not found in the host table a connection to the Internet will be established to do this - which is of course useless. To avoid this enter the client PC in the host table.

Example for PPP Configuration over the serial interface:

        PPP_DEV='com1'
        PPP_SPEED='38400'
        PPP_IPADDR='192.168.4.1'
        PPP_PEER='192.168.4.2'
        PPP_NETWORK='192.168.4.0'
        PPP_NETMASK='255.255.255.0'

and further on in config/base.txt:

        MASQ_NETWORK='192.168.6.0/24 192.168.4.0/24'

The first network number is the one of the ethernet LAN and the second the one of the PPP network.

As long as there is no other net in the 192.168. range in use both nets can be joined in MASQ_NETWORK. This simplifies configuring firewall rules.

In this case use:

        MASQ_NETWORK='192.168.0.0/16'

this masks every net starting with 192.168.

Last thing is to adapt DNS Configuration. Example:

        HOST_5='192.168.4.2 serial-pc'

Do not forget to increment HOST_N!

If the client PC is running Windows you have to configure the dial-up adapter for a PPP connection to the fli4l router.

If a linux client is used create a shell script on the client (i.e. /usr/local/bin/ppp-on):

    #! /bin/sh
    dev='/dev/ttyS0'                    # COM1, for COM2: ttyS1
    speed='38400'                       # speed
    options='defaultroute crtscts'      # options
    myip='192.168.4.2'                  # IP address client
    fli4lip='192.168.4.1'               # IP address fli4l router
    pppd $dev $speed $options $myip:$fli4lip &

In case of problems: man pppd

The fli4l router has to be used as the DNS server on the client if a connection to the Internet is desired. Add two lines to /etc/resolv.conf on the client: the domain used and the ethernet IP address of the fli4l router as name server.

Example:

    search domain.de
    nameserver 192.168.1.4

``domain.de'' res. ``192.168.1.4'' have to be changed to your needs. Important: The IP address has to be the one of fli4l's ethernet card!

A so called null modem cable is used as the physical connection. See appendix for package base for pin wirings.

A (german) Howto for connecting a Windows client with serial PPP can be found at:

http://www.fli4l.de/hilfe/howtos/basteleien/opt-ppp-howto/

© 2001-2022 The fli4l-Team - 7 September 2022